1. Field of the Invention
The present invention relates to certain three-way metal catalysts, and, more especially to such catalysts particularly adopted for the treatment of internal combustion engine exhaust gases.
Consistent herewith, by the expression "three-way catalysts" there are intended those catalysts which effect the oxidation, in particular, of the carbon monoxide and hydrocarbon emissions present in the exhaust gases of internal combustion engines, and also the simultaneous reduction, in particular, of the nitrogen oxide emissions also present in such gases.
2. Description of the Prior Art
In gasoline fueled internal combustion engines, it is conventional to adjust the composition of the exhaust gases to a certain stoichiometric equilibrium such that the catalytic oxidation and reduction of the various constituents thereof result in the formation of water, carbon dioxide and nitrogen. The means typically used for adjusting the composition of the exhaust gas to the desired degree of stoichiometry are, in particular, the continuous adjustment of the air/fuel ratio at the intake of the engine and/or the introduction of additional oxygen upstream of the catalyst.
In practice, this stoichiometric adjustment is never perfect and the composition of the exhaust gas in fact varies over periods of time on the order of one second for a composition containing a relative excess of oxidants (so-called "lean" adjustment) to a composition containing an excess of reducing agents (so-called "rich" adjustment), and vice versa.
In particular, the so-called "lean" adjustment is such that the amounts of oxygen and nitrogen oxide present are greater than those required to effect the oxidation of the carbon monoxide, the hydrocarbons and the hydrogen present. Conversely, and also in particular, the so-called "rich" adjustment is such that the amounts of carbon monoxide, hydrocarbons and hydrogen present are greater than those amounts required to effect the reduction of the oxygen and the nitrogen oxides present.
Catalysts for the treatment of exhaust gases of this type have of course already been proposed to this art. Thus, according to Journal of Catalysis, 56, 321 (1979) and the 6th North American Congress of the Catalysis Society-Chicago-March 1979-communication H.sub.4, three-way catalysts are known which feature cerium oxide and precious metals as the active phase.
And according to published Japanese Application No. 3,062-777, three-way catalysts are also known which feature iron oxide and precious metals as the active phase.
Although, in certain cases, such prior art catalysts possess a sufficient initial activity to satisfy the antipollution standards adopted in countries such as the United States and Japan, these catalysts are characterized by the major disadvantage that they lose a large part of their three-way activity upon their prolonged use in a catalytic converter.